Various motor drive apparatuses are used in many fields. One example is, as illustrated in FIG. 12, a fan motor drive apparatus 1 for driving a fan motor 7, which is a three-phase motor and mounted on a vehicle, by feeding an electric current of a rectangular waveform to each phase based on the pulse width modulation (PWM) control. The fan motor drive apparatus 1 receives, through a signal processing circuit 3, a rotation speed instruction for a fan 6 applied as a PWM signal from a main controller 2 such as an electronic control unit (ECU), generates a voltage signal corresponding to the duty of the PWM signal through conversion, and sends it to a rotation speed instruction conversion circuit 4. The main controller 2 receives an output signal from a water temperature sensor (not shown) that detects the temperature of the water in, for example, a radiator and outputs a rotation speed instruction that is dependent upon the water temperature that is detected.
The rotation speed instruction conversion circuit 4 determines the rotation speed instruction depending upon the voltage signal and sends it to a duty calculation circuit 5. The fan 6 is rotated by a three-phase brushless DC motor 7, and the rotational state of the motor 7 is detected by a position detection circuit 8. The position detection circuit 8 may detect the rotation based on a voltage waveform induced in the winding of the motor 7 (sensorless system), or may use a Hall IC, a resolver or a sensor such as rotary encoder. Therefore, the input terminals of the position detection circuit 8 do not necessarily have to be directly coupled to the winding of the motor 7.
A rotation speed detection circuit 9 calculates the rotation speed of the motor 7 based on a detection signal (rotational position signal of a rotor of the motor 7) output from the position detection circuit 8, and outputs it to the input side of the duty calculation circuit 5. A subtractor 10 calculates a difference between the rotation speed calculated by the rotation speed detection circuit 9 and the rotation speed instruction output from the rotation speed instruction conversion circuit 4, and the result of subtraction is input to the duty calculation circuit 5. The duty instruction calculated by the duty calculation circuit 5 is corrected by a voltage correction circuit 11. The voltage correction circuit 11 detects the voltage of a battery 12 of the vehicle, and adds the above correction to the duty instruction depending upon a variation in the battery voltage.
The corrected duty instruction is output to a PWM signal generation circuit 13. The PWM signal generation circuit 13 generates a PWM signal based on a carrier wave of PWM control generated therein and on the PWM duty determined by the duty calculation circuit 5, and outputs it in common to the input terminals on one side of three AND gates 14U, 14V and 14W.
The detection signals output from the position detection circuit 8 are further applied to a three-phase current distribution circuit 15 which generates 120-degree (120°) current pattern signals based on rectangular waves depending upon the rotational positions of the rotor represented by the detection signals. The 120-degree current pattern signals are output to the input terminals on the other side of AND gates 14U, 14V and 14W, and to a gate drive circuit 16.
While the current pattern signals generated by the three-phase current distribution circuit 15 are assuming the high level, the AND gates 14U, 14V and 14W send the PWM signals generated by the PWM signal generation circuit 13 to the gate drive circuit 16 as high-side signals (high potential side). Further, the current pattern signals directly applied to the gate drive circuit 16 from the three-phase current distribution circuit 15 serve as low-side (low potential side) signals.
An inverter circuit 17 includes, for example, six power MOSFETs (switching elements) 17U, 17V, 17W, 17X, 17Y and 17Z in a three-phase bridge. Gate signals output from the gate drive circuit 16 are applied to the gates of the six FETs 17U to 17Z.
To drive the motor 7 by PWM-controlling the rectangular wave current by using the drive apparatus 1, the ON duty for switching the high-side FETs 17U, 17V and 17W of the inverter circuit 17 is varied to control the rotation speed of the motor 7. In this case, the rotation speed of the motor 7 varies nearly in proportion to the ON duty of the PWM signals, and the rotation speed becomes a maximum at the 100% duty.
Switching elements such as power MOSFETs in the inverter circuit 17 permit an increased current to flow with an increase in the duty ratio of PWM signals and, therefore, generate heat in increased amounts. At 100% duty, however, the switching loss decreases and heat generates in decreased amounts. Unless the elements are capable of withstanding the heat generated at a maximum switching duty, the elements are likely to be destroyed. Therefore, elements or parts for radiating heat tend to become bulky and result in an increase in the cost.
To counter this problem, US 2007/52382A (JP 2006-25565A) and JP 2006-157987A proposes a method to decrease the switching loss by lowering the carrier frequency of PWM signals when the motor is rotating at high speeds (i.e., when the duty ratio is large) or when the detected temperature of the elements is high. However, the carrier frequency if lowered may become audible, and the user may perceive it as the occurrence of offensive noise.
As other method, although the motor 7 is rotated at its normal speed by setting an upper limit (e.g., 80%) which is smaller than 100% as the PWM duty, it is driven to rotate at the 100% duty to lower the generation of heat. Even with this method, however, the rotation speed of the motor is no longer linearly controlled or the rotation speed suddenly changes, when the duty is switched from the upper limit value to 100% producing a bursting sound or permitting an inrush current to flow.